1. Field of the Invention
The invention relates to a process for transporting hydrates of natural gas, petroleum gas or other gases in suspension in a fluid comprising water, one of said gases and a liquid hydrocarbon.
More particularly, it relates to a process in which a composition is used which comprises at least one ester associated with a non-ionic surfactant of the polymerized (dimer and/or trimer) carboxylic acid type.
Gases which form hydrates may comprise at least one hydrocarbon selected from methane, ethane, ethylene, propane, propene, n-butane and isobutane, and possibly H2S and/or CO2.
Said hydrates form when water is in the presence of gas either in the free state or in the dissolved state in a liquid phase such as a liquid hydrocarbon and when the temperature reached by the mixture, in particular water, gas and possibly liquid hydrocarbons, such as oil, drops below the thermodynamic hydrate stability temperature, said temperature being given for a known gas composition when the pressure is fixed.
Hydrate formation is notorious particularly in the gas and oil industry where hydrate formation conditions may occur. To reduce the cost of crude oil and gas production, both from the point of view of investment and from the exploitation point of view, one possible route, in particular for offshore production, is to reduce or do away with the treatments applied to crude oil or gas to be transported from the field to the coast and to leave all or some of the water in the fluid to be transported. Such offshore treatments are generally carried out on a platform located on the surface close to the field, so that the effluent, which is initially hot, can be treated before the thermodynamic hydrate stability conditions are reached due to cooling of the effluent by sea water.
However, as this occurs in practice when the thermodynamic conditions required to form hydrates are satisfied, hydrate agglomeration causes the transport lines to block by creating plugs which prevent the passage of crude oil or gas.
The formation of hydrate plugs may cause production to stop, and thus engender large financial losses. Further, restart of a facility, especially if it involves offshore production or transport, may be lengthy as it is difficult to decompose the hydrates formed. In fact, when the production of a submarine field for natural gas or oil and gas comprising water reaches the surface of the sea bed and is then transported on the sea bottom, the drop in temperature of the effluent means that the thermodynamic conditions for hydrate formation are satisfied; they agglomerate and block the transfer lines. The temperature on the sea bottom may, for example, by 3° C. or 4° C.
Conditions favorable to the formation of hydrates may also occur on land for lines which are above ground or are not deeply buried in the ground when, for example, the ambient air temperature is cold.
2. Description of Related Art
To overcome such disadvantages, the prior art has sought to use products which, when added to fluid, can act as inhibitors by reducing the thermodynamic hydrate stability temperature. They are alcohols such as methanol or glycols such as mono-, di- and tri-ethylene glycol. That solution is very expensive as the quantity of inhibitors to be added may reach 10% to 40% of the water content; further, such alcohols pollute the effluents as such inhibitors are difficult to recover.
Insulation of the transport lines has also been recommended to prevent the temperature of the transported fluid from reaching the hydrate formation temperature under the operating conditions. Again, such a technique is very expensive.
Further, a variety of non-ionic or anionic surfactants have been tested for their hydrate formation retarding ability in a fluid comprising a gas, in particular a hydrocarbon, and water. An example which may be cited is the article by Kuliev et al: “Surfactants Studied as Hydrate Formation Inhibitors”, Gazovoe Delo N° 10, 1972, 17-19, reported in Chemical Abstracts 80, 1974, 98122r.
Further, the use of additives capable of modifying the hydrate formation mechanism has been described since, instead of rapidly agglomerating to form plugs, the hydrates formed disperse in the fluid without agglomerating and without obstructing the lines. In this regard, the Applicant's European patent application EP-A-0 323 774 may be cited, which describes the use of non-ionic amphiphilic compounds selected from esters of polyols and substituted or unsubstituted carboxylic acids, and compounds with an imide function; EP-A-0 323 775, also in the Applicant's name, describes the use of compounds belonging to the fatty acid diethanolamide or fatty acid derivative family; U.S. Pat. No. 4,856,593 describes the use of surfactants such as organic phosphonates, phosphate esters, phosphonic acids, their salts and their esters, inorganic polyphosphates and their esters, as well as polyacrylamides and polyacrylates; and EP-A-0 457 375, which describes the use of anionic surfactants such as alkylarylsulfonic acids and their alkali metal salts.
Amphiphilic compounds obtained by reacting at least one succinic derivative selected from the group formed by polyalkenyl succinic acids and anhydrides on at least one polyethylene glycol monoether have also been proposed to reduce the tendency of natural gas, petroleum gas or other gases to agglomerate (patent application EP-A-0 582 507).